1. Field of the Invention
The present invention relates to a focus control method for focusing a projection lens to form an optical image of an original, like a picture frame on a photo film, on a predetermined image surface, while taking consideration of deviations in focal point of the projection lens with temperature variations. The present invention also relates to an image scan apparatus having a projection lens, to scan an optical image of an original after focusing the projection lens by use of the focusing method.
2. Background Arts
There is a digital print system, in which an image scanner and a digital printer is provided. Scan light is projected toward a photo film with an image, which is focused on a CCD image sensor by use of a focus lens unit. The optical image scanned by the CCD image sensor is photoelectrically converted into digitalized image data. The scanned image is displayed after various image processing, and image data of the scanned image is sent to the digital printer to obtain a print. The digital print system has an advantage that the scanned image can be processed to control the image quality with more flexibility, compared to a conventional system that prints a film image onto a photographic paper directly without image processing.
The digital print system for scanning and recording the image is required to scan the image at high resolution and to reproduce the scanned image with excellent color tone. The digital print system is also needed to cope with various film sizes and various operations like trimming. Then, special optical materials, like fluorite and graded index with low dispersion glass, are used for the focusing lens unit. Usually, such a special optical material has large deviation in refractive index according to temperature change. Thus, focus position of the lens unit is fluctuated, which makes image quality of the scanned image worse.
In order to deal with the above problem, JP-A 9-51408 discloses an image reader which executes a focusing operation to detect a focus position between a pre-scan to determine a scan area and scan condition, and a fine-scan to scan the image at a high resolution. Moreover, JP-A 9-152540 discloses an image inputting lens device which includes a thermometer to detect a temperature of focus lenses, storage means to store standard movement data to a focus position at a standard temperature and correction movement data to move the lens device in accordance with temperature change. Based on these data in the storage means, the lens device calculates a provisional focus position and carries out focusing operation after moving the focus lenses to the provisional focus position. The above two inventions enable an effective focusing operation regardless of fluctuation in temperature of the focusing lenses.
In tile focusing operation, however, the focus position is detected by moving the focus lenses little by little in a search area after moving the lens device to the provisional focus position. Since the focusing operation requires certain time, it is difficult to shorten a whole scan time.